do not worry...it is not as hard as it is propogated....for service SAC codes are applied not hsn code....just go through the items in the sac code and write sac code of that item which is most close to your supplies. in descripttion you can write whatever you were writing before gst.
as far as uploading of invoice is concerned you just have to upload details of invoice on the gst portal, not the actual invoice. you have to do this before 10th of next month....to allow to get familier with this system government have given extra time for submission for details of July month by 1 or 2 month...you should check out....there are various software available online which will help you to mentain your invoices' details....they are paid but most of them are providing free trail for first few months....example cleartax, jiogst etc....cleartax also allowing to practice with fake details...you should check it out